Technical Field
The present invention is an invention relating to a thermal mass flow meter used to measure a mass flow of a fluid, and to a mass flow controller in which the thermal mass flow meter is incorporated.
Description of Related Art
The mass flow controller is widely used in order to control an amount of a process gas to be supplied to a chamber in a manufacturing process of a semiconductor. The mass flow controller comprises a mass flow meter for monitoring a mass flow, a control valve for controlling a mass flow, and a control circuit, and the like. Although there are various kinds of mass flow meters, among others, a thermal mass flow meter has the characteristics of theoretically allowing the correct measurement of a mass flow. For this reason, the thermal mass flow meter is widely prevalent as a mass flow meter incorporated in the mass flow controller for use.
FIG. 1 is a schematic view showing a structure of a sensor section of a thermal mass flow meter. The sensor section 1 of the thermal mass flow meter is provided with a sensor tube 2 in which a fluid flows, with an upstream heater element 3 and a downstream heater element 4 which are formed of heating resistance wires arranged in an outer periphery of the sensor tube 2, and with a plurality of weld terminals 5 to which ends of the heating resistance wires is electrically connected by spot welding. A current is passed through the upstream heater element 3 and the downstream heater element 4, and a fluid flowing in the sensor tube 2 is thereby heated. Since a fluid flows from the upstream toward the downstream and accordingly the heat applied to the fluid also move from the upstream toward the downstream, a temperature distribution of the sensor section 1 is changed and electric resistances of the upstream heater element 3 and the downstream heater element 4 are changed. The changes of the electric resistances are measured, which can detect a mass flow of the fluid flowing in the sensor tube 2.
The ends of the heating resistance wires constituting the upstream heater element 3 and the downstream heater element 4 are electrically connected to surfaces of the plurality of weld terminals 5 by spot welding, respectively. Further to the weld terminals 5, conductive wires 7 for supplying a current to the upstream heater element 3 and the downstream heater element 4 and for measuring a change of an electric resistance value are electrically connected. Thus, the weld terminals 5 function as input-output terminals for supplying a current to the upstream heater element 3 and the downstream heater element 4 and for measuring a change of an electric resistance value. The weld terminal 5 is generally made of phosphor bronze which is easy to be processed and whose electrical conductivity is high, and the like.
Concerning the weld terminal used for the general electrical connection, the configuration of the weld terminal which allows the stable electrical connection by spot welding is previously known. For example, Japanese Patent Application Laid-Open No. H5-166575 discloses a method that when a weld terminal is connected to a conductor of a flat electric wire by spot welding, the weld terminal is pressed on the conductor at an always fixed welding pressure using a presser plate and an excessive current flows through the ground connected to the presser plate, which stabilizes a contact resistance of the conductor and the weld terminal to perform consistently satisfactory spot welding. The weld terminal used for this method is formed by performing nickel plating so as to be 1.5 μm thick on a surface of a substrate made of phosphor bronze as a ground layer, and then performing gold plating so as to be 1 μm thick thereon.
Moreover, for example, Japanese Patent Application Laid-Open No. 2008-218189 discloses a method that when a weld terminal obtained by performing nickel plating on a substrate made of brass as an ground layer and then performing tin plating thereon is spot-welded to a lead frame made of 42 Alloy, a part of the plated layer of the weld terminal is removed and then the weld terminal with the substrate being exposed is spot-welded to the lead frame, which forms a fusion zone in both of the substrate and the lead frame and prevents the weld peeling effectively.